Heretofore, various viscous fluid couplings have been proposed. In the viscous fluid coupling disclosed in Japanese Patent Laid-Open No. 69326/1980, labyrinth grooves are formed on both sides of the rotor, and two working chambers are formed. The partition plate is provided with two holes which place a storage chamber in communication with the working chambers, respectively.
The two holes are opened successively to supply the viscous fluid from the storage chamber into one working chamber and then into the other. This permits the viscous fluid coupling to be switched between three states, i.e., disengaged state, partially engaged state, and fully engaged state. Thus, the rotational speed of the fan rigidly mounted to the outer surface of the coupling can be switched between three different values. Therefore, where the fluid coupling is used to control the rotation of the fan for cooling an automotive engine, the fan is able to cool the engine in response to the temperature of the water cooling the engine.
In the conventional viscous coupling described above, the labyrinth grooves are formed in both side surfaces of the rotor, and the two working chambers are formed. Therefore, the inner surface of the housing and the flat plane of the partition plate are required to have grooves opposite to the labyrinth grooves. This leads to an increase in the manufacturing cost. In addition, it is necessary to fabricate the partition plate from aluminum by die-casting so that it can be cut. For this reason, this part is expensive to fabricate. Furthermore, the thickness, or axial dimension, of the viscous fluid coupling is large, because the labyrinth grooves are formed on both sides of the rotor. This is disadvantageous for installation on a vehicle.